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Old 09-30-2015, 01:13 AM
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Arcanine Arcanine is offline
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Zenith Tuner Problems

I have two Zenith Roundy sets that seem to both suffer the same issue. One has a 25MC33 chassis, and the other a 25LC30 chassis. Schematic wise both appear nearly identical. One has a UHF tuner, the other does not.

I'll differentiate between each set with the two letter variation of each chassis model. LC for the Non-UHF HiFi Set, and MC for the UHF set.

On the LC Chassis, I can at least see an image from the tuner. When I first got it, it seemed like it tuned signals fine until the set was on for awhile, then it just faded away to a blank response less screen. Not even static or signs changing the channel was having any effect. I went back and messed with the AGC control and turned it clockwise pretty far, and got the image to pop back in, with sound and a poor picture. Lots of color defects, unstable image, heavy buzzing, turning AGC back counter clockwise yields nothing, it just goes back to a blank screen with no sound, no static, nothing at all.

I tweaked around with the IF transformers, and wiggled the tubes to make sure they had contact, and I still can't get a good image or sound from it. I checked under the chassis, and all the capacitors in this section seem to be the good ones that are best left alone. No bees, no molded paper. Everything looks good from just a visual inspection.

The MC Chassis can be coaxed to tune a signal, but it looks like the signal is coming from a million miles away. Lot's of static and poor sound.

Per Tom's instruction, I replaced C13, the .15uf cap near where the tuner plugs in to the Chassis, and on the MC, it allowed me to see a signal at least. The LC Chassis appears to have had it replaced already.

What am I doing wrong in attempting to get the tuners working correctly in these sets? They both have an A/V mod, which actually produces a darker image then what the tuner does. I'd like to have the tuners working for the bright, sharp image they produce.

Is there something inside the channel tuner it self that commonly fails? I looked at the sams, and it looks pretty empty besides a couple ceramic caps, and a lot of little wire coils and things I won't dare mess with. Both are also clean. Tuner cleaner has been used on both, and no effect there, either.

Last edited by Arcanine; 09-30-2015 at 01:16 AM.
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Old 09-30-2015, 05:51 AM
consoleguy67 consoleguy67 is offline
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I hate to state the obvious, but have you checked the tubes in the tuners and in the IF stage?
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Old 09-30-2015, 10:26 AM
zeno zeno is offline
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Just asked what was wrong then I scroll down........

Run set with NO signal. Should get a strong high contrast snow
made of dots not streaks. Also a loud waterfall from the audio.

Turn channels. You should get strong static crashes between
channels as the contacts make/break. If you dont its
in the IF. DO NOT TWEAK IF COILS ! Even if it fixed it
you would be just covering up a failed component. Its not
normal drift that much.
If you get the good snow and strong crashes look to the tuner
tubes next. Do not trust a tube checker. Sub in known good
tubes of a brand name. If you dont need tubes in the end all
the tubes are common. 6HA5 used in Zenith from abt 1960 to
the end. 6BZ6, 6EH7, 6EJ7 all used in Zenith and many others.
Start with that.......

73 Zeno
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Old 09-30-2015, 11:30 AM
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Arcanine Arcanine is offline
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Quote:
Originally Posted by zeno View Post
Just asked what was wrong then I scroll down........

Run set with NO signal. Should get a strong high contrast snow
made of dots not streaks. Also a loud waterfall from the audio.

Turn channels. You should get strong static crashes between
channels as the contacts make/break. If you dont its
in the IF. DO NOT TWEAK IF COILS ! Even if it fixed it
you would be just covering up a failed component. Its not
normal drift that much.
If you get the good snow and strong crashes look to the tuner
tubes next. Do not trust a tube checker. Sub in known good
tubes of a brand name. If you dont need tubes in the end all
the tubes are common. 6HA5 used in Zenith from abt 1960 to
the end. 6BZ6, 6EH7, 6EJ7 all used in Zenith and many others.
Start with that.......

73 Zeno
Yeah. Before I started messing with it, once the image went blank, I got no static on the screen, and no sound. No crashes between changing of channels, Even when I turn the AGC to bring snow back to the screen, the sound is soft and weak.

Both sets are like this.
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Old 09-30-2015, 11:42 AM
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Arcanine Arcanine is offline
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I should also mention, with the LC Chassis, when the tuner was working, before it faded off to nothing, I saw a lot of weird garbage on the screen that is not there off the A/V I added in.

I would see Red, Green, and Blue dots, slightly off center, in an even row from top to bottom that would shake a little.

As well as to the far left of the screen on a darker image, I would see a faint vertical bar, in red, green and blue. Both would change and go away if the fine tune was adjusted for a less clear, still color picture.

Generally, the tuner produces nothing unless the AGC is turned very far clock wise. No static, no sound, just a blank screen.
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